1. Field of the Invention
This invention relates to x-ray cross-sectional imaging systems. In a primary application the invention relates to medical imaging systems for providing a cross section representing the amount of an administered material.
2. Description of Prior Art
Cross-sectional imaging systems using x-rays have recently made a significant impact on medical diagnosis. These instruments, made by EMI of England and ACTA in the United States, are used primarily in brain studies. Their relatively long scanning time, of approximately five minutes, limits their use to regions, such as the head, which can be immobilized. Regions involved with respiratory or cardiovascular motions would have a severe loss of resolution due to smearing. The principal reason for the long scanning time is the requirement that the x-ray projections be made over a large number of angles. In addition to the long scanning time, a significant and costly time interval is required to compute the reconstructed image from the projection data. A description of the EMI system is given by J. Ambrose and G. N. Hounsfield in the British Journal of Radiology, Vol. 46, 1973.
These cross-sectional imaging systems can be used to image both density distributions and the presence of an administered contrast material which is selectively taken up in specific areas of the body. Nuclear medicine procedures are also used to image the takeup of materials. In this case, these materials have been radioactively labeled. One method of radioactive labeling involves positron annihilation where a pair of gamma rays are produced each having an energy of 0.51 Mev and travelling in equal and opposite directions. In the case of positron annihilation, the nuclear medicine positron sensitive cameras can measure the line along which the event occurred. Two cameras are positioned to determine the position coordinates of each of the emitted gamma rays. Using a coincidence detection system, when each detector records an event at approximately the same time, the recorded lateral positrons are used to determine the line of occurrence. The determination of the depth dimension is not available. Some attempts have been made to use the time of flight to each detector. However, for 1 cm. resolution in depth, better than 40 picosecond temporal resolution would be required which is extremely difficult to obtain. Another problem with all nuclear medicine procedures, whether or not positron annihilation is used, is the relatively large dosage of radiation given to the patient because of the relatively long half-lives of the radioactive materials.